Fuel cell and related manufacturing method

ABSTRACT

A fuel cell includes an electrode membrane structural body ( 5 ) and a pair of separators ( 9, 9 ) between which the electrode membrane structural body is sandwiched. The electrode membrane structural body includes an electrolyte membrane ( 1 ) and a pair of gas diffusion layers ( 3, 3 ) formed on both surfaces of the electrolyte membrane and serving as electrodes. Each of the pair of separators has gas flow channels ( 13 ) to allow gas to be supplied to associated one of the gas diffusion layers. A porosity of the associated one of the pair of gas diffusion layers at an area outside the gas flow channels is lower than that at an area facing the gas flow channels.

TECHNICAL FIELD

The present invention relates to a fuel cell and a related method, andmore particularly to a polymer electrolyte fuel cell (and a relatedmethod.

BACKGROUND ART

A fuel cell serves to allow fuel gas, containing hydrogen, and oxidativegas, such as air containing oxygen, to electrochemically react with oneanother through an electrolyte to take out electrical energy fromelectrodes formed on both surfaces of the electrolyte. A fuel cellpowered vehicle equipped with such a fuel cell is one which is installedwith a hydrogen storage device such as a high pressure hydrogen tank, aliquid hydrogen tank and a hydrogen absorbing alloy tank to allowresulting hydrogen gas and air containing oxygen to be delivered to afuel cell for reaction for thereby taking out electric energy by which amotor connected to drive wheels is driven, with exhausted substancebeing water to create a clean vehicle.

Especially, PEFC, which has a solid polymer electrolyte, operates at alow temperature while providing an ease of handling and, hence, PEFC isfocused on attention as an electric power supply of an electricallypowered vehicle.

A cell forming an electric power generation unit of PEFC takes the formof a structure wherein an electrode membrane structural body,constructed of a solid polymer electrolyte membrane and, an anode gasdiffusion layer and a cathode gas diffusion layer that are formed onboth sides of the solid polymer electrolyte membrane is sandwichedbetween a pair of separators.

Japanese Patent Application Laid-Open Publication No. 2001-319667relates to a gas diffusion layer, a separator and a gasket in a cell ofa fuel cell and contemplates to restrict a gap between the gas diffusionlayer and the gasket surrounding the gas diffusion layer for therebyimproving a sealing property between the gas diffusion layer and agasket portion of the gasket.

DISCLOSURE OFINVENTION

However, according to studies conducted by the present inventor, whiledepending on the fuel cell with such a structure, an improvement inperformance can be expected by decreasing the gap to the minimum betweenthe gas diffusion layer and the gasket located at the outside the gasdiffusion layer, it is conceivable that due to the gas diffusion layerper se being porous, reaction gas flows through the interior of the gasdispersing layer disposed between the gas flow channels, formed in theseparator, and the gasket with no contribution to electric powergeneration. This leads to the occurrence in which a portion of gassupplied to a gas inlet of the separator wastefully flows to a gasoutlet without contributing to electric power generating reaction with aresultant drop in an electric power generating efficiency.

Further, with such a cell structure, when locating the gas diffusionlayer on the separator, because of the reason that the gas diffusionlayer should entirely cover the gas flow channel region formed on theseparator and it is necessary for the gas diffusion layer per se to beaccurately positioned, it is considered that there are manyprobabilities in which the gas diffusion layer must be set to be largerthan the reacting area.

FIG. 8 is a partial cross sectional view schematically illustrating astructure of a fuel cell 80 which is also described later in conjunctionwith a Comparative Example and is studied based on such a structure.Also, for the sake of convenience of description, only one unit cell ofthe fuel cell 80 is shown in an exploded status.

As shown in FIG. 8, an electrode membrane structural body 105 iscomprised of an electrolyte membrane 101 and two gas diffusion layers103, 103 formed on both surfaces of the electrolyte membrane 101. Theelectrode membrane structural body 105 is sandwiched between theseparators 109, 109, that is, a cathode separator 109 a and an anodeseparator 109 b, and a gasket 107 is disposed between the electrodemembrane structural body 105 and each of the separators 109 a, 109 b.

Here, it is conceived that a distance (shortest distance) L between thegas flow channels 113, on each of the separators 109 a, 109 b, and thegasket 107 is set to a value in the order of several millimeters toprovide a non-reacting region for precluding gas from flowing through anarea outside the reacting region without reaction.

In the meanwhile, since a width of each gas flow channel lies in a valuein the order of several millimeters, such a non-reacting region lies inat least a width greater than that of each gas flow channel and thus asurface area of the reacting region is inevitably caused to decrease bysuch an extent. This leads to a drop in output power of the fuel cell inthe first place. Moreover, even if such a non-reacting region isprovided, it becomes hard to improve flow control of gas passing throughthe porous gas diffusion layer.

The present invention has been made upon such studies conducted by thepresent inventor and particularly has an object to provide a fuel cell,which has a structure made of gas diffusion layers, gaskets andseparators wherein introduced gas is prevented from wastefully flowingthrough an area outside a reacting region to allow introduced gas to beentirely and efficiently subjected to reaction, and a related method.

According to one aspect of the present invention, a fuel cell comprises:an electrode membrane structural body provided with an electrolytemembrane; and a pair of gas diffusion layers formed on both surfaces ofthe electrolyte membrane and serving as electrodes, respectively; and apair of separators between which the electrode membrane structural bodyis sandwiched, each of the pair of the separators having gas flowchannels that allow gas to be supplied to associated one of the pair ofgas diffusion layers, and a porosity of the associated one of the pairof gas diffusion layers at an area outside the gas flow channels islower than a porosity of the associated one of the pair of gas diffusionlayers at an area facing the gas flow channels.

In other word, according to another aspect of the present invention, afuel cell comprises: an electrode membrane structural body providedwith: an electrolyte membrane; and a pair of gas diffusion layers formedon both surfaces of the electrolyte membrane and serving as electrodes,respectively; a pair of separators between which the electrode membranestructural body is sandwiched, each of the pair of the separators havinggas flow channels that allow gas to be supplied to associated one of thepair of gas diffusion layers; and lowering means for lowering a porosityof the associated one of the pair of gas diffusion layers at an areaoutside the gas flow channels than a porosity of the associated one ofthe pair of gas diffusion layers at an area facing the gas flowchannels.

In the meanwhile, according to another aspect of the present invention,there is provided a method of manufacturing a fuel cell, which methodcomprises: preparing an electrode membrane structural body providedwith: an electrolyte membrane; and a pair of gas diffusion layers formedon both surfaces of the electrolyte membrane and serving as electrodes,respectively, and sandwiching the electrode membrane structural bodybetween a pair of separators each of which has gas flow channels thatallow gas to be supplied to associated one of the pair of gas diffusionlayers, a porosity of the associated one of the pair of gas diffusionlayers at an area outside the gas flow channels being lower than aporosity of the associated one of the pair of gas diffusion layers at anarea fig the gas flow channels.

Other and further features, advantages, and benefits of the presentinvention will become more apparent from the following description takenin conjunction with the following drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a partial cross sectional view schematically showing a fuelcell of a first embodiment according to the present invention;

FIG. 2 is a partial cross sectional view schematically showing a fuelcell of a second embodiment according to the present invention;

FIG. 3 is a partial cross sectional view schematically showing a fuelcell of a third embodiment according to the present invention;

FIG. 4 is a partial cross sectional view schematically showing a fuelcell of a fourth embodiment according to the present invention;

FIG. 5 is a partial cross sectional view schematically showing anelectrode membrane structural body of a fuel cell of a fifth embodimentaccording to the present invention;

FIG. 6 is a view showing a characteristic of a gas diffusion layer interms of a surface pressure encountered representatively in the firstembodiment of the present invention;

FIG. 7 is a characteristic of electric voltage in terms of electriccurrent showing experimental results of the first and fourth embodimentsof the present invention and a Comparative Example; and

FIG. 8 is a partial cross sectional view schematically showing astructure of a fuel cell of the Comparative Example.

BEST MODE FOR CARRYING OUT THE INVENTION

A fuel cell and a related method of each of embodiments according to thepresent invention and a Comparative Example are described hereunder indetail with suitable reference to the accompanying drawings.

FIRST EMBODIMENT

First, referring mainly to FIG. 1, a fuel cell 10 and its related methodof a first embodiment according to the present invention are describedin detail. Incidentally, for the sake of convenience of description,only one unit cell of the fuel cell 10 is shown in an exploded status.Further, the unit cell of the fuel cell 10, shown in the presently filedembodiment, is of a structure which has a reacting region G with asurface area of 150 mm×150 mm and separators each formed of a graphiteplate, with a size of 200 mm×200 mm×2.5 mm, which is formed with gasflow charnels and coolant flow channels, with an electrolyte membraneand a gas diffusion layer having thickness of 30 μm and 280 μm,respectively.

As shown in FIG. 1, an electrode membrane structural body 5 is formed ofan electrolyte membrane 1 and two gas diffusion layers 3, 3 formed onboth surfaces of the electrolyte membrane 1. The gas diffusion layers 3,3 are made of porous members each having pores, with one of thediffusion layers serving as an anode electrode supplied with fuel gascontaining hydrogen while the other gas diffusion layer serves as acathode electrode supplied with oxidative gas (oxidizer gas) such as aircontaining oxygen.

Gaskets 7, 7 are interposed between the electrode membrane structuralbody 5 and the separators 9, 9 (one of which is a cathode separator 109a and the other of which is an anode cathode separator 109 b) topreferably make a round at an interface area therebetween while theelectrode membrane structural body 5 is sandwiched at both sides thereofbetween the separators 9, 9, thereby forming the fuel cell 10 in theform of the unit cell.

More particularly, each separator 9 has a main surface formed withrecess-like gas flow channels 13 by cutting and has the other surface,opposite to the main surface, formed with coolant flow channels 15 bycutting. Gasket recesses 11 are formed on the respective separators atfurther outer areas than that in which the gas flow channels 13 areformed.

Further, formed on each separator 9 in contact with the outermost endportion of the gas flow channels 13 is a convex portion 21 thatprotrudes from the main surface of the separator 9 toward the electrodemembrane structural body 5 by a height t The convex portion 21 is formedin a way to make a round on the separator 9 at an area further insidethan a terminal portion of the gas diffusion layer 3 of the electrodemembrane structural body 5. That is, an area in which the convex portion21 is fanned may be sufficient to be set at a position between theoutermost end portion of the gas flow channel 13 and the gasket 7 suchthat the convex portion 21 surrounds a gas flow channel placement regionS, in which the gas flow channels 13 are formed, as viewed in adirection parallel to a page surface (as viewed from the above in FIG.1).

Here, the porous member for use in each gas diffusion layer 3 is used ina status with an appropriate porosity as a result of compressionexecuted in a direction of a thickness of the cell during assembly ofthe fuel cell 1 and also with a required conductivity.

FIG. 6 is a view showing the relationship between a surface pressure ofeach gas diffusion layer 3 and a thickness thereof in the presentlyfiled embodiment. In the figure, the abscissa axis represents thesurface pressure p of the gas diffusion layer 3 and the ordinate axisrepresents the thickness t_(D) of the gas diffusion layer 3.Incidentally, such a relationship is similar in each of the embodiments1 to 5.

As shown in FIG. 6, it is understood that as the surface pressure pexerted to each gas diffusion layer 3 gradually increases, the pores inthis component are crushed to cause the thickness t_(D) of the gasdiffusion layer 3 to be thinned and as the gas diffusion layer 3 becomesthinner, the porosity the gas diffusion layer 3 decreases.

More particularly, the porous gas diffusion layer 3 exhibits a tendencyin that although during a time interval in which the surface pressure premains low, the thickness t_(D) of the gas diffusion layer decreases ininverse proportion to an increase in the surface pressure p, a furtherincrease in the surface pressure p results in a decrease in thethickness to with a smaller decreasing rate. And, as the surfacepressure p exceeds a certain value of B, the gas diffusion layer 3finishes crushing whereby a tendency appears in that even when thesurface pressure p is further increased, almost, no variation takesplace in the thickness t_(D).

Here, it is supposed that in the porous gas diffusion layer 3, thesurface pressure A is exerted to the gas diffusion layer 3 at thereacting region (electric power generating region) G corresponding tothe gas flow channel placement region S and the thickness reaches avalue of t_(A).

However, at the convex portion 21 of the separator 9, the gas diffusionlayer 3 is compressed in excess by a height t (in particular, t is setto 30 μm) and, as a result the gas diffusion layer 3 has a thicknessequal to a value of t_(A)−t at an area against which the convex portion21 abuts.

Accordingly, the gas diffusion layer 3 facing the convex portion 21 issufficiently compressed to a further extent than that of the reactingregion G facing the gas flow channel placement region S in which the gasflow channels 13 are located, resulting in a lower porosity than that ofthe reacting region. Incidentally, the amount of the gas diffusion layer3 to be compressed is preferred to lie at an extent in that a pressureloss of gas flowing through an area of the gas diffusion layer 3correspondingly located outside the gas flow channels 13 is larger thana pressure loss of gas flowing through the other area of the gasdiffusion layer 3 remaining in the reacting region G More preferably, itis good for the porous structure of the area of the gas diffusion layer3 correspondingly located outside the gas flow channels 13 to have athickness t_(B), at which crushing of the gas diffusion layer 3 fishesat an extent not to allow gas to flow. Also, a width or the like of theconvex portion 21 may be suitably altered depending on the surfacepressure p resulting when the convex portion 21 abuts against the gasdiffusion layer 3.

Further, since the convex portions 21 of both the separators 9 for theanode and the cathode are disposed in symmetry with respect to theelectrolyte membrane 1 (symmetric with respect to the surface of theelectrolyte membrane 1 extending perpendicular to the page surface ofFIG. 1), the gas diffusion layers 3 are equally compressed on both sidesof the electrolyte membrane 1, thereby enabling adjustments so as todecrease the porosities of the gas diffusion layers 3. That is, it ispossible to accomplish adjustments so as to decease the porosities ofthe gas diffusion layers 3 without causing the electrolyte membrane 1from being distorted due to excessive external forces.

With the structure of the presently filed embodiment set forth above,due to an ability of suppressing the occurrence in which a portion ofintroduced reaction gas does not flow along the gas flow channels formedon the separators and flows in the gas diffusion layers at an area apartfrom the reacting region, a substantially entire part of introduced gascan be reacted, resulting in a capability of improving a performance ofthe fuel cell.

Furthermore, in contrast to a method wherein the terminal portions ofthe porous members are concealed with resin, with the presently filedembodiment, once the height of the convex portion with respect to themain surface of the separator is determined, the separator can beefficiently fabricated by such as press forming without a need for anywasteful time and extra expenses.

Incidentally, of course, no limitation is intended to the number ofpieces of the unit cells, dimensions and configurations of theparticular arrangements to be used in the presently filed embodiment,and it is, of course, possible to apply other conditions to the fuelcell of the presently filed embodiment provided that similar functionsare obtained.

SECOND EMBODIMENT

Next, referring mainly to FIG. 2, a fuel cell 20 and its related methodof a second embodiment according to he present invention are describedin detail. Also, for the sake of convenience of description, only oneunit cell of the fuel cell 20 is shown in an exploded status, with onlyone of two separators 9, 9 for the anode and the cathode placed insurface symmetry with respect to the electrode membrane structural body5 being illustrated Also, the same component parts as those of the firstembodiment bear the same reference numerals and description is suitablysimplified or omitted.

As shown in FIG. 2, the presently filed embodiment differs from thefirst embodiment in that, in contrast to the convex portion 21 of thefirst embodiment shown in FIG. 1, a convex portion 31 has a shape inwhich corner portions (edge portions) are formed with roundconfigurations R, respectively, and both embodiments are identical inother structure.

With the structure of the presently filed embodiment, due to the edgeportions being removed from the convex portion 31 with the roundconfigurations R, even if the gas diffusion layer 3 is made of a fragileporous member such as a carbon paper, it is possible to avoid the gasdiffusion layer 3 from being deteriorated in function due to cracks orthe like caused in the member.

Additionally, due to the presence of reduction in a surface contact areaof the gas diffusion layer as compared to that of the first embodiment,even if the fuel cell stack is exerted with and compressed by the samemagnitude of load as that exerted to the fuel cell stack in the firstembodiment, the gas diffusion layer 3 undergoes a further increased loadat a portion facing the convex portion 31, enabling that portion to havea further decreased porosity.

THIRD EMBODIMENT

Next, referring mainly to FIG. 3, a fuel cell 30 and its related methodof a third embodiment according to the present invention are describedin detail. Also, for the sake of convenience of description, only oneunit cell of the fuel cell 30 is shown in an exploded status, with onlyone of two separators 9, 9 for the anode and the cathode placed insurface symmetry with respect to the electrode membrane structural body5 being illustrated. Also, the same component parts as those of thefirst embodiment bear the same reference numerals and description issuitably simplified or omitted.

As shown in FIG. 3, the presently filed embodiment differs from thefirst embodiment in that, in contrast to the convex portion 21 of thefirst embodiment shown in FIG. 1, a convex portion 41 has a slopedsurface with a height differing between an area closer to the gas flowchannels 13 and the other area closer to the gasket 7 such that asurface facing the gas diffusion layer 3 has a lower height at an areacloser to the gasket 7 than that of the other, and the residualstructure is identical as that of the first embodiment. Also, in thepresently filed embodiment, the gas diffusion layer 3 is made of acarbon cloth that is formed in a porous member.

With the structure of the presently filed embodiment, the presence ofthe highest portion of the convex portion 41 being located closer to thegas flow channels 13 enables the gas diffusion layer 3 to becollectively exerted with the load, thereby enabling introduced reactiongas to be further effectively avoided from flowing out from the gas flowchannels 13 toward an area closer to the gasket 7.

FOURTH EMBODIMENT

Next, referring mainly to FIG. 4, a fuel cell 40 and its related methodof a fourth embodiment according to the present invention are describedin detail. Also, for the sake of convenience of description, only oneunit cell of the fuel cell 40 is shown in an exploded status, with onlyone of two separators 9, 9 for the anode and the cathode placed insurface symmetry with respect to the electrode membrane structural body5 being illustrated. Also, the same component parts as those of thefirst embodiment bear the same reference numerals and description issuitably simplified or omitted.

As shown in FIG. 4, the presently filed embodiment differs from thefirst embodiment in that the convex portion 21, such as the one of thefirst embodiment shown in FIG. 1, is not provided whereas an insulationmember 51 is disposed inside the gasket 7 to have a flat surface facingthe gas diffusion layer 3 and an insulation member 52 is disposedoutside the gasket 7 to have a flat facing the gas diffusion layer 3such that the gasket 7 is sandwiched by the insulation member 51 and theinsulation member 52, and the residual structure is identical as thestructure of the first embodiment.

More particularly, a thickness of the insulation member 52 disposedoutside the gasket 7 is set to be equal to a thickness of the gasdiffusion layer 3 in a case where a unit cell is exerted with apredetermined load. Moreover, the thickness of the insulation member 51disposed inside the gasket 7 is set in the same way as that used in thefirst embodiment with reference to FIG. 6.

Such insulation members 51, 52 also have functions to prevent the anodeseparator and the cathode separator from being short-circuited in theunit cell and may preferably have heat-resistant properties in respectof an operating temperature (in the vicinity of 100° C.) of the fuelcell, hydrolysis-resistant properties in respect of humidifyingoperation and acid resisting properties derived from the electrolytemembrane, while no care need to be undertaken to employ any ofthermosetting resin and thermoplastic resin.

Further, since the insulation members 51 are provided at the anode andcathode sides in surface symmetry with respect to the electrode membranestructural body 5, the gas diffusion layers 3 of the electrode membranestructural body 5 are equally compressed, thereby enabling the porosityto be decreased for adjustment. That is, no probability occurs in whichthe electrolyte membrane 1 encounters distortion due to excessiveexternal forces.

With the structure of the presently filed embodiment, thus, in contrastto a situation to be conceivable where although the gas diffusion layeris compressed to a certain specified value in use, this results in adifference in the degree of compression of the gas diffusion layerdepending on how the load is exerted thereto to cause a difference inthe degree of gas diffusion with a resultant increased inequality inperformances of the unit cells, the insulation members between theelectrode membrane structural bodies are used as thickness adjustingmembers for the gas diffusion layers.

Accordingly, due to a capability of using the insulation members betweenthe electrode membrane structural bodies as the thickness adjustingmembers for the gas diffusion layers, setting the thickness of theinsulation layer to an extent equal to a compressed thickness of the gasdiffusion layer, in order to enable a compressed extent of the gasdiffusion layer to have a certain thickness however hard the gasdiffusion layer is compressed, makes it possible to ensure all of theunit cells to have the gas diffusion layers each with a uniformthickness.

FIFTH EMBODIMENT

Next referring mainly to FIG. 5, a fuel cell 50 and its related methodof a fifth embodiment according to the present invention are describedin detail. Also, for the sake of convenience of description, only oneelectrode membrane structural body is shown. Also, the same componentparts as those of the first embodiment bear the same reference numeralsand description is suitably simplified or omitted.

As shown in FIG. 5, the electrode membrane structural body 50 iscomprised of the electrolyte membrane 1 and two gas diffusion layers 3formed on both sides of the electrolyte membrane 1.

More particularly, the gas diffusion layers 3 of the electrode membranestructural body 50 have respective end portions 61 that are formed bycompressing the same in opposite directions (in vertically oppositedirections in the figure) perpendicular to both of the main surfaces.And, the electrode membrane structural body 50 obtained as a result ofsuch compression is applied to the structure of the first embodiment,thereby permitting the fuel cell to be assembled. Also, it is, ofcourse, to be noted that the electrode membrane structural body 50having such end portions 61 is available for application to thestructures of the second to fourth embodiments.

When compressing the end portion 61, as seen in the compression curveshown in FIG. 6, it is preferable for the end portion 61 to becompressed to a thickness t_(B) until no further variation takes placein a thickness t_(D) of the gas diffusion layer 3. According to thisfeature, the load to be applied to the unit cells of the fuel cell to beassembled as a stack is sufficient to be only set such that thethickness t_(D) of the gas diffusion layer 3 equals t_(A).

More particularly, methanol solution of thermosetting resin is preparedand impregnated in the gas diffusion layer 3 whereupon parts of the endportions 61 of the gas diffusion layers 3 are imparted with a load witha surface pressure of 2 MPa at a surrounding temperature of 120° C. by acompressing press for thereby hardening resin, thereby obtaining the gasdiffusion layer 3 having the compressed end portions 61. Incidentally,it may be arranged such that the end portions 61 of the gas diffusionlayers 3 are preliminarily formed in more increased thickness and,thereafter, the end portions 61 are compressed to obtain decreasedporosities, while on the other hand, making an attempt to set eachthickness corresponding to the electric power generating region to failn a value of t_(A). In such case, there is no need for forming theconvex portions 21, 31, 41, 51 which are previously mentioned.

Further, since the compressed areas of the end portions 61 of the gasdiffusion layers 3 are formed in surface symmetry with respect to theelectrolyte membrane 1, the gas diffusion layers 3 are equallycompressed with respect to the electrolyte membrane 1, obtainingdecreased porosities. This enables the electrolyte membrane to beeffectively avoided from suffering from distortion due to undesiredexternal forces.

With the structure of the presently filed embodiment thus mentioned, theuse of the gas diffusion layers, of the electrode membrane structuralbody, preliminarily compressed at the regions in which no contributiontakes place for electric power generation, makes it possible toeffectively avoid gas introduced to the fuel cell from flowing into aregion which is unavailable in contribution to electric powergeneration.

COMPARATIVE EXAMPLE

Next, referring mainly to FIG. 8, Comparative Example that was studiedin the presently filed embodiment is described in detail Also, for thesake of convenience of description, only one unit cell of the fuel cell80 is illustrated in the exploded status. Also, the same component partsas those of the first embodiment bear the same reference numerals anddescription is suitably simplified or omitted.

As shown in FIG. 8, in the present Comparative Example, the electrodemembrane structural body 105 is comprised of the electrolyte membrane101, and the two gas diffusion layers 103 formed on both sides of theelectrolyte membrane 101. Disposed between the electrode membranestructural body 105 and the separators 109, 109, i.e., the cathodeseparator 109 a and the anode separator 109 b is the gasket 107, and theelectrode membrane structural body 105 is sandwiched between the cathodeseparator 109 a and the anode separator 109 b. In this ComparativeExample, the outermost distance L between the gas flow channel 13 andthe gasket 107 is set to a value of 4.5 mm, and a spaces between the gasdiffusion layer 103 and the gasket 107 is set to a value of 1.2 mm. And,the unit cell stack with such a condition is compressed at a surfacepressure of 1.0 MPa, thereby obtaining a fuel cell that is stacked.

FIG. 7 is a view for typically showing an electric current voltagecharacteristic illustration showing experimental results obtained inrespect of an I-V characteristic as a result of electric powergeneration of the fuel cells of the first and fourth embodiments of thepresent invention and the Comparative Example. In the figure, theabscissa axis represents electric current I, the ordinate axisrepresents electric voltage V, a curve a represents the I-Vcharacteristic of the fuel cells 10, 40 of the first and fourthembodiments, and a curve b represents the I-V characteristic of the fuelcell 80 of the Comparative Example. Operating conditions of therespective fuel cells 10, 40, 80 were based on setting each celltemperature to 80° C., using hydrogen gas as reaction gas for the anodeand air as reaction gas for the cathode and humidifying such that bothhydrogen gas and air reach a steam content of 80% of saturated steamcontent at a cell temperature. Also, the gas diffusion layers 3, 103 aremade of carbon papers.

As shown in FIG. 7, it is understood that the I-V characteristic of thefuel cells 10, 40 of the first and fourth embodiments are furtherimproved than that of the fuel cell 80 of Comparative Example.

According to the structures of the respective embodiments of the presentinvention described above, since the porosity of the area, which islocated outside the gas flow channels of the separator of the gasdiffusion layer of the electrode membrane structural body is madesmaller than the porosity of the area facing the gas flow channels, theamount of gas flowing through the gas diffusion layer at the areaoutside the gas flow channels to be unavailable in contribution toreaction can be restricted with a resultant advantageous effect toincrease an efficiency of the fuel cell.

The entire content of a Patent Application No. TOKIJGAN 2002-382139 witha filing date of Dec. 27, 2002 in Japan is hereby incorporated byreference.

Although the invention has been described above by reference to certainembodiments of the invention, the invention is not limited to theembodiments described above. Modifications and variations of theembodiments described above will occur to those skilled in the art, inlight of the teachings. The scope of the invention is defined withreference to the following claims.

INDUSTRIAL APPLICATION

As set forth above, in the fuel cell and the related method of thepresent invention, the porosities of the gas diffusion layers of theelectrode membrane structural body at the areas located outside the gasflow channels of the separators are set to be smaller than theporosities of the areas facing the gas flow channels. With such astructure, it becomes possible to restrict the amount of gas flowingthrough the gas diffusion layers at the arm outside the gas flowchannels to be unavailable in contribution to reaction for therebyincreasing an efficiency of the fuel cell, and therefore a wide range ofapplication is expected involving a fuel cell powered automobile.

1. A fuel cell comprising: an electrode membrane structural bodyprovided with: an electrolyte membrane; and a pair of gas diffusionlayers formed on both surfaces of the electrolyte membrane and servingas electrodes, respectively; and a pair of separators between which theelectrode membrane structural body is sandwiched, each of the pair ofthe separators having gas flow channels that allow gas to be supplied toassociated one of the pair of gas diffusion layers, and a porosity ofthe associated one of the pair of gas diffusion layers at an areaoutside the gas flow channels is lower than a porosity of the associatedone of the pair of gas diffusion layers at an area facing the gas flowchannels.
 2. The fuel cell according to claim 1, wherein each of thepair of separators has a convex portion formed at the area outside thegas flow channels.
 3. The fuel cell according to claim 2, wherein theconvex portion is held in pressured contact with the associated one ofthe pair of gas diffusion layers.
 4. The fuel cell according to claim 2,wherein a corner of the convex portion is formed in a round portion. 5.The fuel cell according to claim 2, wherein the convex portion has asloped surface angled with respect to the associated one of the pair ofgas diffusion layers.
 6. The fuel cell according to claim 5, wherein aheight of the convex portion increases as the convex portion is close tothe gas flow channels.
 7. The fuel cell according to claim 2, whereinthe convex portion surrounds the gas flow channels.
 8. The fuel cellaccording to claim 2, further comprising a gasket intervening betweeneach of the pair of separators and the associated one of the pair of gasdiffusion layers, wherein the convex portion is positioned between thegas flow channels and the gasket.
 9. The fuel cell according to claim 1,further comprising a pressing member intervening between the electrodemembrane structural body and each of the pair of separators to press theassociated one of the pair of gas diffusion layers.
 10. The fuel cellaccording to claim 9, wherein the pressing member presses the associatedone of the pair of gas diffusion layers at the area outside the gas flowchannels.
 11. The fuel cell according to claim 9, wherein the pressingmember includes an electrically insulating member.
 12. The fuel cellaccording to claim 9, further comprising a restricting member thatrestricts the pressing member.
 13. The fuel cell according to claim 12,further comprising a gasket intervening between each of the pair ofseparators and the associated one of the pair of gas diffusion layers,wherein the restricting member is located at an area outside thepressing member with the gasket being located therebetween.
 14. The fuelcell according to claim 1, wherein the associated one of the pair of gasdiffusion layers at the area outside the gas flow channels ispreliminary compressed.
 15. The fuel cell according to claim 1, whereinthe porosity of the associated one of the pair of gas diffusion layersis distributed in surface symmetry with respect to the electrolytemembrane.
 16. A fuel cell comprising: an electrode membrane structuralbody provided with: an electrolyte membrane; and a pair of gas diffusionlayers formed on both surfaces of the electrolyte membrane and servingas electrodes, respectively; a pair of separators between which theelectrode membrane structural body is sandwiched, each of the pair ofthe separators having gas flow channels that allow gas to be supplied toassociated one of the pair of gas diffusion layers; and lowering meansfor lowering a porosity of the associated one of the pair of gasdiffusion layers at an area outside the gas flow channels than aporosity of the associated one of the pair of gas diffusion layers at anarea facing the gas flow channels.
 17. A method of manufacturing a fuelcell, comprising: preparing an electrode membrane structural bodyprovided with: an electrolyte membrane; and a pair of gas diffusionlayers formed on both surfaces of the electrolyte membrane and servingas electrodes, respectively; and sandwiching the electrode membranestructural body between a pair of separators each of which has gas flowchannels that allow gas to be supplied to associated one of the pair ofgas diffusion layers, a porosity of the associated one of the pair ofgas diffusion layers at an area outside the gas flow channels beinglower than a porosity of the associated one of the pair of gas diffusionlayers at an area facing the gas flow channels.